A compressor body of an oil-cooled type screw compressor is provided with a rotor casing for storing a pair of male and female screw rotors meshing with each other. While rotor shafts on both ends of the pair of male and female screw rotors are supported by radial bearings, a pair of tilting pad thrust bearings for receiving a thrust force generated on the screw rotors are provided on one of rotor-shaft end portions of the pair of respective male and female screw rotors. The tilting pad thrust bearings (referred to as thrust bearings hereinafter) are provided at positions where a disk-shaped thrust member, which fits over one of the rotor-shaft end portions of the pair of respective male and female screw rotors, is between the thrust bearings. Therefore, the thrust bearings are in contact with sliding surfaces of the thrust member to receive a thrust force transmitted from the screw rotors to the thrust member. As such an oil-cooled type screw compressor which can reduce a thrust force, a conventional example described in a patent document 1 is known, for example.
A description will now be given of an outline of the oil-cooled type screw compressor according to this conventional example referring to accompanying drawings. FIG. 6 is a diagram showing an overall configuration of the oil-cooled type screw compressor according to the conventional example, FIG. 7 is a diagram showing an internal configuration of a compressor body of the oil-cooled type screw compressor according to the conventional example, and FIG. 8 is an enlarged view of a portion of thrust bearings and a balance piston of the compressor body of the oil-cooled type screw compressor according to the conventional example.
The illustrated oil-cooled type screw compressor includes: a compressor body 53, one side of which is connected to a suction flow passage 51, and the other side of which is connected to a discharge flow passage 52; and an oil feed flow passage 57 which connects an oil sump unit 55 at a bottom portion of an oil separator/collector 54 provided on the discharge flow passage 52 and main lubricated portions inside the compressor body 53, via an oil pump 56. Between a downstream side of the oil separator/collector 54 and an upstream side of the oil pump 56, a uniform pressure flow passage 58 branches, and communicates with the compressor body 53 as described later.
As shown in FIG. 7, the compressor body 53 includes a casing, which is not shown, and a pair of male and female screw rotors 61 disposed in the casing and meshing with each other. The screw rotors 61 are rotatably supported by the radial bearings 63 at rotor shafts 62 extending from each of the screw rotors 61. In FIG. 7, the left side is a suction side, the right side is a discharge side, two arrows on the left side indicate an inflow of a suction gas, and an arrow on the right side indicates an outflow of a discharge gas. Moreover, reference numerals Ps and Pd in the drawing denote a suction pressure of the suction gas and a discharge pressure of the discharge gas respectively.
Moreover, in case of the compressor shown in FIG. 7, the rotor shaft 62 of one rotor (male rotor) 61 which extends leftward includes an input shaft 65 which receives a rotational driving force by a motor, which is not shown. Further, the thrust bearings 66 are provided on the rotor shaft 62 to the right side of the radial bearing 63 on the discharge side of each of the rotors 61. On the other hand, a disc-shaped thrust plate 64 is fitted near the other end of each of the rotor shafts 62, and a pair of thrust bearings 66 for receiving the thrust force generated on the screw rotors 61 is provided on both sides of the thrust plate 64. The thrust bearings 66 are in contact with sliding surfaces of each of the thrust plates 64, and receives the thrust force transmitted from the screw rotor 61 to the thrust plate 64.
Moreover, a balance piston 67 is fixed at the other end of each of the rotor shafts 62. A partition flange 81 is provided between the balance piston 67 and the thrust bearings 66. Within an internal peripheral portion of the partition wall 81, shaft seal means 82 having an airtight/fluidtight property is fitted to each of the rotor shafts 62. This shaft seal means 82 blocks the pressure between a space AS storing the thrust bearings 66 and a space BS storing the balance piston 67 while permitting the rotation of the corresponding rotor shaft 62. Therefore, the space BS is separated from the other components such as the input shaft 65, thrust bearings 66, and radial bearings 63. In case of this oil-cooled type screw compressor, as described above, the compressor body 53 has a single stage configuration. Even in case of an oil-cooled type screw compressor provided with the multiple staged compressor body 53, same configuration is possible.
In the compressor body 53 of the oil-cooled type screw compressor having this configuration, as shown in FIG. 7, the suction pressure Ps from the suction flow passage 51 is introduced from the side of the input shaft 65 to the space AS, and the oil at a discharge pressure Pd+α (note that α>0) is fed from the oil feed flow passage 57 to the radial bearings 63. On the other hand, the oil supplied from the uniform pressure flow passage 58 to the balance piston 67 side and having pressure Pd which is adjusted to be equivalent to the discharge pressure Pd is led to the surface, on the thrust bearing 66 side, of the balance piston 67 in the space BS.
As shown in FIGS. 6 and 7, basically, the suction flow passage 51 is at the suction pressure Ps, the discharge flow passage 52 is at the discharge pressure Pd, a primary side of the oil pump 56 on the oil feed flow passage 57 is at the discharge pressure Pd, and a secondary side of the oil pump 56 is at the oil feed pressure Pd+α (note that aα>0), though there is some pressure change. And a relationship in magnitude of the respective pressures is represented as:Ps<Pd<Pd+α
Thus, as described above, the introduction of the suction pressure Ps and the discharge pressure Pd+α into the space As, and the introduction of the pressure-adjusted oil into the space Bs largely contribute to the reduction of the thrust force, which has been a problem.
Moreover, in the conventional example shown in FIGS. 6 to 8, not only the thrust force is reduced, it can also provide measures against an problematic anti-thrust load immediately after a startup, during an unload operation, and the like. In other words, in case of the small load of the compressor, namely in case of the small thrust force, such as immediately after the startup, and during the unload operation, a force which is larger than and against the force applied to the screw rotors 61 in a direction from the discharge side to the suction side, that is so-called anti-thrust load, may be applied. However, in the conventional example shown in FIG. 6 to 8, the partition wall 81 for blocking the pressure is provided between the thrust bearings 66 and the balance pistons 67, and the uniform pressure flow passage 58 for introducing, without pressurizing, the oil in the oil sump unit to the space, on the partition wall 81 side, of the balance pistons 67 is provided. Therefore, such anti-thrust load can be efficiently prevented.
Assuming that the outer diameter of the balance pistons 67 is D, and the shaft diameter of the balance pistons 67 is d, a force:F=(D2−d2)·(n/4)Pd acts on each of the balance pistons 67.
Since the force F is proportional to the discharge pressure Pd, the force F becomes small when force applied to the screw rotors 61 in the direction from the discharge side to the suction side is small, such as immediately after the startup of the compressor body 53, and during the unload operation. Thus, an excessive anti-thrust force is not generated, and even if the bearings are worn, a collision of the screw rotors 61 with a wall portion of a rotor chamber is avoided. In this way, in the conventional example, the pressure receiving areas of the balance pistons 67 are increased, and also the thrust bearings 66 having a large load capacity are employed to prevent the generation of a state of the anti-thrust load.
However, since the thrust bearings 64 are provided at positions remote from the screw rotors 61, and the balance pistons 67 are provided at further remote positions, this configuration is not sufficiently compact though it is more compact than earlier “compressor bodies”.
[Patent Document 1] Japanese Patent No. 3766725